1. Field of the Invention
The present invention relates to a cathode-ray tube apparatus used in a TV receiver, a computer display, and the like. The present invention also relates to a velocity modulation coil apparatus to be mounted on the cathode-ray tube apparatus.
2. Description of Related Art
As one method for realizing higher image quality in a TV receiver, for example, the enhancement of the edge of an image is known. In order to enhance an edge of an image, a velocity modulation coil is used. The velocity modulation coil is provided at a neck of a cathode-ray tube or in the vicinity thereof, and generates a magnetic field in a vertical direction to modulate the horizontal scanning velocity of an electron beam, thereby enhancing the edge of an image (e.g., see JP 57(1982)-45650 U and JP6(1994)-283113 A).
In the color cathode-ray tube apparatus, along with the increase in a diameter of an electron beam spot on a phosphor screen ascribed to the recent enlargement of a screen, the increase in an anode voltage for higher brightness, or the increased flatness of a front panel, there is a demand for a further higher intensity in the magnetic field for enhancing an edge of an image.
In order to satisfy the above-mentioned demand, a color cathode-ray tube apparatus has been proposed that is capable of increasing the intensity of the magnetic field acting on an electron beam without increasing the amount of a current that flows through a velocity modulation coil and without increasing the winding number of the velocity modulation coil (e.g., see JP 6(1994)-283113 A and JP2003-116019 A).
In the color cathode-ray tube apparatus described in JP 6(1994)-283113 A, a pair of magnetic substances are placed in upper and lower portions of respective electron beam passage apertures of R, G, and B provided at a fifth grid (G5 electrode) of an electron gun housed in a neck, and a pair of velocity modulation coils are placed at positions on an outer circumferential surface of the neck corresponding to the G5 electrode.
In JP 2003-116019 A, a pair of velocity modulation coils and a pair of magnetic substances are placed so as to be opposed to each other in a vertical direction on an outer circumferential surface of a neck. Herein, the magnetic substance is placed in the vicinity of the center of a loop of the velocity modulation coil.
In JP 6(1994)-283113 A and JP 2003-116019 A, due to the above-mentioned configurations, a magnetic flux generated by a pair of velocity modulation coils is collected by a pair of magnetic substances so as to be concentrated in an electron beam passage region. Therefore, the intensity of a magnetic field contributing to the velocity modulation of the electron beam can be increased.
However, in the color cathode-ray tube apparatus described in the above-mentioned JP 57(1982)-45650 U, the velocity modulation coil, and a horizontal deflection coil and a vertical deflection coil are placed so as to be overlapped with each other along the tube axis direction. Therefore, a magnetic field generated by the velocity modulation coil and a deflection magnetic field generated by the horizontal deflection coil and the vertical deflection coil interfere with each other to cause the degradation of image quality (so-called ringing).
Furthermore, in the color cathode-ray tube apparatus described in the above-mentioned JP 6(1994)-283113 A, due to the loss caused by an eddy current generated on the surface of the electrode (G5 electrode) that is a metal component, the intensity of a magnetic field generated in the electron beam passage region in the G5 electrode is low. Thus, even if the magnetic substances are attached to the G5 electrode to allow an originally weak magnetic flux to be collected, a satisfactory increase of the intensity of a magnetic field cannot be expected. That is, in JP 6(1994)-283113 A, the sensitivity of velocity modulation (velocity modulation amount of an electron beam with respect to a current input to the velocity modulation coil) cannot be enhanced sufficiently. Furthermore, the magnetic substances are placed at the G5 electrode constituting the electron gun in the neck, so that the component assembly man-hour increases, resulting in an increase in a cost.
Furthermore, even if the magnetic substance is placed in the vicinity of the center of a loop of the velocity modulation coil as in the above-mentioned JP 2003-116019 A, the magnetic resistance with respect to the magnetic field generated by the velocity modulation coil cannot be reduced sufficiently. Consequently, a sufficient velocity modulation effect cannot be obtained.
On the other hand, a strong magnetic field also can be obtained by increasing the winding number of the velocity modulation coil instead of placing the magnetic substances as in JP 6(1994)-283113 A and JP 2003-116019 A. However, in this case, the impedance of the velocity modulation coil increases, making it necessary to apply a large power to the velocity modulation coil, which leads to an increase in a cost of a driving circuit.
Thus, a procedure of improving the effect of enhancing an edge of an image by the velocity modulation coil without increasing a driving power has not been realized.